1. Field of the Invention
The present application relates to WLAN (Wireless Local Area Network) receivers and corresponding methods and integrated circuit chips, and in particular to the header detection in such receivers.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist.
Examples of these extensions are the IEEE 802.11a, 802.11b and 802.11g standards. The 802.11a specification applies to wireless ATM (Asynchronous Transfer Mode) systems and is used in access hubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. It uses a modulation scheme known as Orthogonal Frequency Division Multiplexing (OFDM) that makes possible data speeds as high as 54 Mbps, but most commonly, communications take place at 6 Mbps, 12 Mbps, or 24 Mbps. The 802.11b standard uses a modulation method known as Complementary Code Keying (CCK) which allows high data rates and is less susceptible to multi-path propagation interference. The 802.11g standard can use data rates of up to 54 Mbps in the 2.4 GHz frequency band using OFDM. Since both 802.11g and 802.11b operate in the 2.4 GHz frequency band, they are completely inter-operable. The 802.11g standard defines CCK-OFDM as optional transmit mode that combines the access modes of 802.11a and 802.11b, and which can support transmission rates of up to 22 Mbps.
In any transmit mode, a WLAN receiver needs to detect the headers in the received signals. For this purpose, conventional WLAN receivers convert the incoming analog signals to digital signals and perform some digital signal processing on the converted signals to detect the headers. This is an approach which has been shown to usually work properly, but under certain circumstances, the conventional header detection schemes are of insufficient precision and accuracy and work sometimes inefficiently. Moreover, the power consumption involved with detecting headers in conventional receivers is rather high.